1. Field of the Invention
This invention relates to axial flow pumps.
More particularly, this invention relates to axial flow waterjet pumps wherein different horsepower inputs are adapted to a single stator housing by trimming as required the axial impeller section and the pump nozzle section. This permits a single basic design to be used for a wide range of applications with resulting economy.
2. Description of the Prior Art
There are many patents that attempt to deal with fluid flow rate control of a basic design. For example, U.S. Pat. No. 3,664,001 provides for adjustment of the compressor flow rate. The free edges of the unshrouded impeller are machined off, thus reducing the blade height and moving the operating point to the lower desired flow rate. To cover the impeller correctly, new stationary shrouds have to be supplied. In addition, the free edges of the diffuser blades have to be machined off in such a fashion as to match the impeller exit. This is made possible by the installation of separate annular diffusers. The invention thus allows for changing the design flow rate of a compressor (or pump) and does not deal with a change in horsepower input utilizing an unchanged stator housing as does the present invention.
Swiss Pat. No. 368,160 discloses a design concept that adjusts the operating point of an axial flow pump by reducing the tip diameter of the axial pump rotor and filling the gap between the housing and rotor with an insert. The instant invention again deals with horsepower changes that do not alter the pump housing or the stators; only the impeller configuration.
Any waterjet propulsion system driven by a fixed speed drive such as a diesel engine can be adjusted to any power level by either changing speed by a gear change or changing the impeller and stator vane housing. It is desirable, particularly in commercial applications, to eliminate the costly gearbox. Without the gearbox, the system can be matched by changing the pump size in accordance with the engine power level. This approach is expensive because the major pump parts must be changed. This is true because pump horsepower is a function of flow and headrise where, if the same headrise is assumed, the flow drops from the 100% level at 1000 HP to the 80% level at 800 HP. As a consequence, the impeller inlet as well as exit velocity triangles change requiring new hardware. If, in this example, no stator hardware change is adapted, a low efficiency would result, coupled with a fast stator vane deterioration due to cavitation damage.
Other combinations of impeller headrise and flow rate can be selected to satisfy the horsepower conditions, some of which are in the prior art. All of them, however, require new pump hardware for each horsepower level. The one exception, the subject of this invention, allows the same pump hardware to be used for a wide horsepower range, requiring only a trimming modification of the axial impeller and nozzle opening. The instant invention reduces the necessary hardware changes to a minimum, eliminates the gearbox for fixed speed engine applications, and allows the same pump hardware to be used for a wide horsepower range at a constant speed while maximizing the stator vane life.
Axial impeller waterjet pumps require a stator vane section downstream of the impeller in order to remove the whirl for optimum jet flow conditions. This stator is subject to erosion due to cavitation unless the stator vane inlet angle matches the flow angle exiting the trailing edge of the impeller blade. As horsepower is changed, the axial impeller, as well as the stator vane section must be redesigned in order to result in cavitation-free stator flow (unless the pump speed is changed as by use of a gearbox). The invention eliminates the redesign by trimming the trailing edge of the blades of the axial impeller such that the velocity direction entering the stator vane section is unchanged. Thus, the pump rotor diameter and housing containing the fixed stator vanes remain unchanged; thereby allowing the use of a single pump housing for different horsepower ranges in a family of waterjets. The pump nozzle is trimmed to match the required flow rate for each power level. One nozzle casting is used.
The instant invention will accommodate horepower levels, for example, between 500 to 1000 HP at a constant pump speed without experiencing stator vane cavitation damage by applying the following ground rules:
(a) The stator inlet tip diameter must be constant for all horsepowers.
(b) The stator inlet hub diameter must be constant for all horsepowers.
(c) The stator vane shape is constant for all horsepowers.
(d) The ratio of impeller discharge flow coefficient over head coefficient must be kept in a range which results in stator vane cavitation margin (nearly constant) for all horsepower levels.
None of the foregoing prior art patents accomplish these goals, while the present invention accomplishes all these parameters.